1. Field of the Invention
The present invention relates to an alloy sheet for electronic devices having high etching performance, and particularly to an alloy sheet suitable for the materials of shadow masks on color cathode ray tubes and of IC lead frames.
2. Description of the Related Arts
Fe--Ni alloys have been used as a material for shadow masks on color cathode ray tubes and for IC lead frames. The Fe--Ni alloys have a significantly low thermal expansion coefficient compared with low carbon steels which have been conventionally used as the materials for electronic devices. For this reason, for example, a shadow mask prepared from Fe--Ni alloy sheet rarely raises a problem of color phase shift caused by thermal expansion even if it is heated by an electron beam.
Fe--Ni alloy sheets used for shadow masks and IC lead frames are subjected to photo-etching process. Conventional Fe--Ni alloy sheets have, however, a disadvantage of inferior etching performance to low carbon steel. In concrete terms, Fe--Ni alloys show considerably poor corrosion to etching liquid and have a large crystal grain size compared with low carbon steels. Consequently, when Fe--Ni alloy sheets are etched to provide pierced holes, the distribution of hole diameters and the hole shape become dispersive. With the disadvantage, Fe--Ni alloy sheets serving as shadow masks tend to generate a blurred periphery on the masks when a light is penetrated through the fine holes prepared by etching. Furthermore, the brightness of masks penetrated by light is poorer than that of masks made of low carbon steels. In particular, high definition masks having fine pitch and fine holes, which have increasingly been requested by the electronics market, likely induce the above described problem which markedly degrades the quality of color cathode ray tubes. In addition, recent color cathode ray tubes strongly demand a high screen brightness, and a inferior mask brightness reduces the competitiveness of products. Regarding the matarials for IC lead frames, the movement toward high density (high integration) of IC demands a fine pitch of the pin arrangement on a lead frame. Since the conventional Fe--Ni alloys have the problems described above, they can not respond to the request for a fine pitch of the pin arrangement. Adding to the problem, conventional Fe--Ni alloys have a disadvantage of inferior performance of plating after etching.
Several technologies to solve the problem on etching performance of Fe--Ni alloys have been proposed. They include the following.
(1) Japanese examined Patent publication No. 2-9665 discloses an alloy sheet having a gathering degree of {100} plane of 35% or more on the surface of sheet as an Invar alloy sheet which realizes high definition and uniform etching.
(2) Japanese unexamined Patent publication No. 62-243782 discloses a method for producing a Fe--Ni Inver alloy which improves etching speed and reduces a blurred periphery of a pierced hole, the alloy having the {100} plane on its surface and a surface roughness of Ra in a range of 0.2 to 0.7 .mu.m and a Sm of 100 .mu.m or less and a crystal grain size number of 8.0 or more.
(3) Japanese unexamined Patent publication No. 2-270941 discloses a method for producing a Fe--Ni Invar alloy which improves etching speed, the alloy having a degree of {200} plane of 50% or more on its surface. The alloy also has 0.007 wt. % or less C, and impurities of 0.005 wt. % or less P and 0.005 wt. % S, the other impurities being 0.10 wt. % or less.
However, the technology of (1) cannot prevent the generation of a blurred periphery on a prepared shadow mask and is inferior in the brightness of the mask to conventional masks made of low carbon steel, though the technology improves the precision and uniformity of etching. The technology of (2) is inferior in the brightness of the prepared shadow mask to that made of low carbon steel, though the etching speed increases and the production of blurred periphery of pierced hole is improved.
The technology of (3) raises a problem of excessive side etching on prepared IC lead frames and of poor processing accuracy as lead frames.
These three technologies the have problem of inferior plating performance of IC lead frames processed by etching. For instance, when an IC lead frame obtained by the technology of (3) is subjected to solder plating, abnormal growth of acicular crystals called "whisker" occurs, which raises a quality problem.